1. Field of the Invention
The present invention relates to a semiconductor process, and more particularly, to a method of forming a phase shift mask for the semiconductor wafer.
2. Description of the Prior Art
In order to define the circuit of the integrated circuits in a semiconductor process, the designed pattern is initially formed on the mask and then transferred onto the semiconductor wafer by lithography process. A prior art phase shift mask comprises a flat glass substrate, a phase shifter layer, and a chromium (Cr) layer with the pattern on it. The pattern on the Cr layer is formed by the exposure and the development processes. When transferring the pattern of the phase shift mask onto the semiconductor wafer, the phase shifter layer can generate 180.degree. phase shift angle for the light penetrating through, which can reduce the pattern boundary vibration so as to accurately transfer the pattern onto the semiconductor wafer.
Please refer to FIG. 1 to FIG. 4. FIG. 1 to FIG. 4 are schematic diagrams of a prior art method of forming a phase shift mask 20. A prior art method of forming a phase shift mask 20 is performed on a mask substrate 10 made of quartz. According to the designed pattern, the mask substrate 10 comprises a predetermined region A and a predetermined region B. When forming the phase shift mask 20, a phase shifter layer 12 and a shield layer 14 are subsequently formed on the mask substrate 10. Next, a first lithography process is performed to form a first photo-resist layer 16 on the surface of the predetermined region A of the mask substrate 10, as shown in FIG. 1. Then, a first etching process is performed to vertically remove the shield layer 14 not covered by the first photo-resist layer 16, and then remove the first photo-resist layer 16 completely, as shown in FIG. 2. And next, a second lithography process is performed to form a second photo-resist layer 18 on the surface of the predetermined region B of the mask substrate 10, as shown in FIG. 3. Then, a second etching process is performed to vertically remove the phase shifter layer 12 not covered by the second photo-resist layer 18, and then the second photo-resist layer is completely removed, as shown in FIG. 4, so as to complete the phase shift mask 20.
The prior art method of forming the phase shift mask 20 has to perform the first lithography process to form the first photo-resist layer 16 and remove the shield layer 14 surrounding the predetermined region A, and then perform the second lithography process to form the second photo-resist layer 18 and remove the phase shifter layer 12 surrounding the predetermined region B. Since the prior art method has to repeat the lithography process, the alignment accuracy of the pattern finally formed on the phase mask 20 may be reduced which may affect the quality of the subsequent semiconductor process. Besides, repeating the lithography process may raise the processing cost, and reduce the profit of the semiconductor product.